"About 1750 Black went to Edinburgh University to complete his medical studies... The attention of medical men was directed at this time to the action of lime-water as a remedy for stone in the bladder. All the medicines which were of any avail in mitigating the pain attendant on this disease more or less resembled the "caustic ley of the soap-boilers" (...caustic potash or soda). These caustic medicines were mostly prepared by the action of quicklime on some other substance, and quicklime was generally supposed to derive its caustic, or corrosive properties from the fire which was used in changing ordinary into quicklime. When quicklime was heated with "fixed alkalis" (i.e. with potassium or ), it changed these substances into caustic bodies which had a corrosive action on animal matter; hence it was concluded that the quicklime had derived a "power"—or some said had derived "igneous matter"—from the fire, and had communicated this to the fixed alkalis, which thereby acquired the property of corroding animal matter."